There's two factors in play here... First, it depends on your router's processor... i.e. the CPU in old linksys WRT54G's were around 200MHz... A Netgear WNR3500L is 450MHz... Other routers are even faster...

Second, it depends on the internet up/down speeds at each location. If your upload speed is only 1Mb or less then building a dedicated PC to run your OpenVPN server makes little sense.

There is also a 'fast-io' command you can use for open-vpn which supposedly gives a little boost in performance.